Early detection of threat agents can be a great benefit in either disease prophylaxis or therapy before symptoms appear or worsen. Every species, strain or toxin of a microbe contains unique surface ligands. Using molecular engineering and/or immunological techniques, receptor molecules, such as antibodies or aptimers, can be isolated that will bind to these ligands with high specificity. Examples of ligand/receptor detection systems for identifying threat agents include, but are not limited to, detection of a pathogenic agent such as a microbe by an antibody, or of a toxin by an antibody, or detection of an antibody in blood by another antibody, or binding of a chemical toxin, such as nerve gas, to its receptor.
U.S. Pat. No. 6,171,802 (the '802 Patent) by Woolverton, Niehaus, Doane, Lavrentovich, Schmidt and Signs, the entire disclosure of which is fully incorporated herein by reference, discloses a system for detecting selective binding of a ligand to a receptor and producing an amplified optical signal as the formed receptor-ligand aggregate distorts the orientation of a surrounding liquid crystal matrix. The system comprises a receptor (e.g., mono-specific antibody), a ligand (e.g., the specific pathogenic agent that binds to the antibody), and a liquid crystalline material. Under baseline conditions, a uniform nematic liquid crystal slab is positioned between two glass plates whose surfaces are treated to uniformly align the liquid crystal (FIG. 8). The resulting cassette is viewed between two polarizers, a director and an analyzer, with the analyzer oriented 90° out of phase with the director. The easy axis orientation of the liquid crystal matrix (i.e., the direction of positional orientation of the liquid crystal) is aligned with the director and light is projected through the device. Since the analyzer is perpendicular to the polarization of light, light is not transmitted across the uniformly aligned detection system and the cassette appears dark. Under assay conditions, however, the receptor-ligand aggregates, also referred to as inclusion bodies, become embedded in the liquid crystal matrix and induct director distortions in that matrix. The magnitude of director distortion is determined by the balance of the liquid crystal-to-particle anchoring energy versus the elastic energy of director distortions in the liquid crystal bulk. Both anchoring energy and elastic energy are affected by particle size such that individual ligands or receptors are too small to induce detectable distortions, i.e., particle diameter (d) is less than the critical detectable diameter (dc). Larger particles, ligand aggregates for example, that exceed the critical diameter will cause the adjacent liquid crystal to deviate from it's original uniform orientation. The distorted zone will permit light transmission and will appear as an optically detectable bright spot.
U.S. Patent Application Publication No. 2002/0052002 by Niehaus, Woolverton, Lavrentovich, Ishikawa and Doane, the entire disclosure of which is fully incorporated herein by reference, discloses an additional system and methods for amplifying receptor-ligand binding to enhance detection signals created in the liquid crystal. The system and method comprise a generally spherical particle capable of binding to the desired ligand. The diameter of the individual spherical particle is less than the critical diameter (dc) and thus does not initiate a detectable optical signal. In one embodiment, the generally spherical particle is coated with a receptor that specifically binds to the ligand (e.g. a microsphere coated with an antibody specific for the ligand). In another embodiment, the system comprises an antibody specific for the ligand, such antibody not being attached to the generally spherical particle. In this latter embodiment, a generally spherical particle is also provided that is coated with a receptor for the antibody (e.g., a microsphere coated with anti-immunoglobulin). The system also comprises a liquid crystalline material. In the absence of a ligand to which the antibody binds, the liquid crystalline material assumes an ordered orientation, and polarized light directed through the system, similar to the description above, does not reach a light detector. In the presence of a ligand to which the antibody can bind, however, the microsphere-antibody-ligand complex becomes large enough to exceed the critical diameter, distorts the liquid crystalline material, and allows local transmission of light to the photodetector. The generally spherical particle thus significantly enhances formation of detectable (d>>dc) microsphere-antibody-ligand aggregates when ligand concentrations are low.
U.S. Pat. No. 7,060,225 (the '225 Patent) by Niehaus, the entire disclosure of which is fully incorporated herein by reference, discloses a self-contained device, called an assay cassette, for rapid, safe, and automatic detection of biohazardous ligands. The cassette comprises one or more mixing chambers configured for mixing various assay components with an introduced sample, such assay components being a receptor (e.g., an antibody) capable of binding to the ligand, an optional generally spherical particle capable of binding to the receptor (e.g., microspheres coated with antibody); and a liquid crystalline material. The device is configured to provide for turbulent flow of the sample and assay components such that there is mixing between the sample and assay components. The device also comprises detection chambers into which the sample that has been mixed with the assay components (i.e., mixed sample) flows. The device is also configured to provide for laminar flow of the mixed sample into the detection chambers such that the liquid crystalline material assumes a uniform alignment with the director except when adjacent to an inclusion body whose diameter exceeds the critical diameter (d>>dc), e.g., surrounding immune aggregates. The local liquid crystal distortions allow transmission of spots of polarized light through the detection chamber for identification by a photodetector.